1. Field of the Invention
The present invention relates to an electronically steerable beam-forming system. Specifically, the present invention is a system for forming independently steerable wide band cardioid radiation patterns.
2. Description of the Related Art
The tactical electromagnetic environment at a high frequency (HF) level has been found to be extremely hostile to communication. Examples of the factors which render this environment hostile are: limitations in the receiver due to poor sensitivity; multipath and multimode propagation; co-channel and adjacent channel interferors; and jammers, whether friendly or hostile.
In an effort to jam interferors or to receive a desired signal in the presence of a jamming signal, various beam-forming techniques have been developed to minimize unwanted directional signals or to maximize desired signals in the presence of noise. These conventional techniques have concentrated on signal processing at the intermediate frequency (IF) level.
However, signal processing at the IF level is usually limited to a narrow frequency band. Furthermore, some approaches to develop nulls automatically have centered on the use of linear feedback techniques to find, for example, the location of jamming emitters.
The aforementioned IF processing technique requires a reasonably high signal-to-noise (S/N) ratio typically on the order of six or higher decibels (dBs), and does not work well if the S/N ratio drops to three dBs or less. Signal processing at the IF level imposes limitations also on the band width of the developed nulls, thus decreasing the utility and effectiveness of receivers employing this technique.
For example, if wide band deep nulls could be created directly from the signals induced on an antenna array, before the received signal is transmitted to the receiver, the band limitations imposed by conventional IF processing techniques would be overcome.
Indeed, analog radio frequency (RF) processing would provide wide bandwidth advantages which are not currently attainable at IF levels with digital processing approaches due primarily to the bandwidth/sampling limitations imposed by analog-to-digital (A/D) converters, and the weight, space and power requirements on board real host platforms. The broadband nature of analog spatial processing techniques makes it ideally suited for applications in broadband systems.